The application is based on application No. JP 11-181094 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to an improved filter, illumination device and illumination method, and as a specific application thereof, to a filter, illumination device and illumination method that are suitable for an optical print head used in a color printer, for example.
2. Description of the Related Art
As a solid scanning type optical recording device, a PLZT optical shutter array is known. A PLZT optical shutter array is formed of PLZT ceramic having a superior electrooptical effect, and through appropriate selection of the operating voltage, high-speed control of whether incident light passes through or is blocked by the array may be performed irrespective of the wavelength of the incident light. Moreover, because a PLZT optical shutter array can optically record information in a very small area, it may be used in an optical print head. It is especially effective in an optical print head for a color printer that uses color-photosensitive silver halide paper as the recording medium.
There are various methods employed by the color printer optical print head using this PLZT optical shutter array (hereinafter xe2x80x98PLZT color print headxe2x80x99). In one method, a single PLZT optical shutter array is used, and the color of the incident light that strikes the PLZT optical shutter array is sequentially alternated according to a timing sequence. FIG. 6 shows the basic construction of a PLZT color print head using this method, which offers high performance.
First, the construction of the PLZT color print head shown in FIG. 6 will be explained. The PLZT color print head shown in FIG. 6 comprises a halogen lamp 1, which works as a light source, a heat filter 2, a color filter constructed as a color wheel 3, an integrator 4 that makes the intensity of the light uniform, an optical fiber light guide 5 that guides the light and converts the light leaving the light exit outlet into a linear beam, and an optical shutter unit comprising a polarizer 6, a PLZT optical shutter array 7, an analyzer 8 and a rod lens array 9. The integrator 4 is located at a position at which it can receive the light emitted from the halogen lamp 1, and the heat filter 2 and the color filter comprising light-permeable optical filters are located between the integrator 4 and the halogen lamp 1. The color filter comprises a round color wheel 3, in which color filters that respectively allow blue (B), green (G) and red (R) light to pass through are located in an area having a fan configuration that includes straight lines radiating outward from the center of the wheel and representing the radius of the circle. The light inlet 4a of the integrator 4 has a size and position that allow it to receive only light that passes through one of the multiple areas into which the color wheel 3 is divided. The light that passes through the B, G and R color filters that sequentially alternate according to a timing sequence through the rotation of the color wheel 3 strikes the integrator 4.
The operation of the PLZT color print head shown in FIG. 6 will now be explained. The white light emitted from the halogen lamp 1 comprising the light source (rated voltage=24V, rated power=250 W) first passes through the heat filter 2. Light that is harmful to the color development of the color-photosensitive silver halide paper (i.e., UV light and infrared light) is screened out from this white light by means of this heat filter 2. The white light that passes through the heat filter 2 then passes through the color wheel 3 that rotates in one direction at a high fixed speed (12,000 rpm or higher). FIG. 7 shows the positions of the filters in the color wheel 3 as seen from the side of the heat filter 2. The color wheel 3 is divided into six segments. In the color wheel 3 are color filters 3B, 3G and 3R, which allow only blue (B), green (G) and red (R) light to pass through, respectively, and there are two filters of each type. The filters are positioned in a concyclic fashion (in other words, each filter has a fan configuration), with each filter located opposite its matching filter around the circumference of the round color wheel 3. Because the central angle xcex8 formed by each color filter 3B, 3G and 3R is 60xc2x0, when the color wheel 3 rotates, the color filters 3B, 3G and 3R that allow the light from the halogen lamp 1 to pass through alternate in a prescribed time sequence, and as a result, light of a single color is emitted from the color wheel 3 according to an alternating time-based sequence (i.e., Bxe2x86x92Gxe2x86x92Rxe2x86x92B, etc.).
The light inlet 4a of the integrator 4 is positioned relative to the center of the color wheel 3 in the manner shown by the solid lines in FIG. 7, and appears to move around the circumference of the color wheel as the color wheel rotates. (In fact, the light inlet 4a is fixed and the filter rotates, and as described above, the light striking the light inlet 4a is sequentially alternated according to a timing sequence.) The light that strikes the light inlet 4a at the time that the light inlet 4a is positioned such that it extends onto the next filter (or the component that serves as a border) cannot be used to expose the recording medium. Therefore, only the light that passes through the filter within the range of the central angle xcex1 (the range that excludes the area within the central angle xcex8 where the color wheel is switching from one filter to another) in FIG. 7 is used as light to perform actual exposure.
The light that exits the color wheel 3 enters the integrator 4 from the square light inlet 4a, and by passing through the integrator 4, is converted to uniform illumination light. The light passing through the integrator 4 strikes the optical fiber light guide 5. This optical fiber light guide 5 comprises multiple plastic optical fibers that are bound such that together they comprise a cylindrical configuration at the light inlet end, whereas the light outlets are aligned in a linear fashion. Therefore, the light striking the optical fiber light guide 5 is converted into straight-line illumination light having a high level of brightness at the light exit outlet. The light converted into straight-line illumination light illuminates the PLZT optical shutter array 7 after passing through the polarizer 6. After being modulated by the PLZT optical shutter array 7 and passing through the analyzer 8 and rod lens array 9 (such as the SELFOC lens manufactured by Nihon Panel Glass Co., Ltd.), it reaches a recording medium not shown in the drawing, such as color-photosensitive silver halide paper.
The PLZT optical shutter array 7 has an array construction in which approximately 60 xcexcm optical modulation elements are aligned at a 400 dpi pitch. The PLZT optical shutter array 7 selectively allows the light to pass through or be blocked on a pixel-by-pixel basis through electrical drive control in response to the color of the incident light, based on exposed image information. Therefore, the light of each color that is modulated by the PLZT optical shutter array 7 is made to strike the recording medium by the rod lens array 9 in accordance with a timing sequence, thereby exposing the recording medium. At the same time, the recording medium moves in the secondary scanning direction Y (the direction perpendicular to the shutter array line direction X) relative to the PLZT color print head. In this way, a two-dimensional color image is recorded on the recording medium.
The PLZT color print head can print images at a print speed of one inch per second. However, to make it compatible with faster printers, the print speed of the print head must be increased. In order to do this, the alternating of the exposure colors must be speeded up by increasing the speed of rotation of color wheel 3 or the number of segments. However, there are limits to the speed with which the exposure colors alternate, and from the standpoint of the need to supply light energy (i.e., an amount of light) corresponding to the spectral sensitivity of the color-photosensitive silver halide paper for that color, it is preferred that the filters be alternated with high efficiency. The reason for this will be explained with reference to FIGS. 8 and 9.
The graph in FIG. 8 shows the spectral characteristics of the white light after it passes through the heat filter 2. In FIG. 8, the thick solid line shows the spectral characteristic when the lamp voltage is 20V, the dashed line shows the spectral characteristic when the lamp voltage is 18V, the dotted line shows the spectral characteristic when the lamp voltage is 15V, the dashed/one dot line shows the spectral characteristic when the lamp voltage is 12V, the dashed/two dot line shows the spectral characteristic when the lamp voltage is 10V, and the fine solid line shows the spectral characteristic when the lamp voltage is 5V. In each case, the curve can be seen to be hill-shaped, with the peak being reached at around 600 nm. With regard to absolute light amount, E+00 indicates xc3x9710xc2x0, while Exe2x88x9201 indicatesxc3x9710xe2x88x921.
On the one hand, the graph in FIG. 9 shows the spectral sensitivity curve for general color-photosensitive silver halide paper. The sensitivity (logarithm) is the inverse of the exposure light amount (erg/cm2) necessary to obtain a prescribed darkness. The effective exposure time is 0.5 seconds, and the developer is RA-4. It can be seen from FIG. 9 that the yellow color development layer is sensitive to the single-color light of blue (B), that the magenta color development layer is sensitive to the single color light of green (G), and that the cyan color development layer is sensitive to the single color light of red (R), and that the sensitivity of the color-sensitivc silver halide paper decreases significantly in the order of Bxe2x86x92Gxe2x86x92R. In other words, the amount of energy needed to expose the color-photosensitive silver halide paper increases in the order of R, G, B.
On the other hand, the light emission spectrum of the light source shows only a single peak in the area of green (G) light, as seen from FIG. 8. In other words, the spectral distribution of the R component that requires a large amount of light energy to expose the recording medium is relatively weak in the light source. Therefore, it can be seen that it will not be easy to obtain sufficient light energy to ensure cyan color development (i.e., R exposure). This problem could be eliminated by increasing the power of the halogen lamp 1, but there are obviously limits to this approach. The reason for this is that increasing the power of the light source also increases the effect of the heat rays and increases power consumption.
The present invention was created in consideration of the situation described above. Its object is to provide an improved filter or illumination device. In other words, its object is to provide an illumination device by which the amount of light necessary to perform exposure may be efficiently obtained from the light source. Another object is to improve the efficiency of exposure of the recording medium without increasing the amount of light emitted from the light source. More specifically, its object is to provide an illumination device to efficiently perform exposure in accordance with the spectral sensitivity of the recording medium. Another object is to provide an illumination device that reduces the amount of time that exposure of the recording medium cannot be performed due to the alternating of filters.
In order to obtain these and other objects, the illumination device of one aspect of the present invention is an illumination device comprising a light source, multiple color filters with different light permeability or reflectance characteristics located in a continuous fashion, a drive mechanism constructed such that the multiple color filters that either reflect the light from the light source or allow it to pass through are sequentially alternated based on a timing sequence, and a lighting member that takes in from the light inlet the light that passes through or is reflected from the color filters and guides it in a prescribed direction, wherein the time required for the light passing through or being reflected from at least one of the multiple color filters to be taken in by the lighting member is different from the time required for light passing through or being reflected from the other color filters.
The light illumination device of another aspect of the present invention is an illumination device comprising a light source, a rotatable round color wheel that has multiple color filters aligned around the circumference of the color wheel that have different light permeability or reflectance characteristics, wherein when the color wheel rotates, the color filters that either reflect the light from the light source or allow it to pass through sequentially alternate according to a timing sequence, a drive mechanism that drives the color wheel to rotate, and a lighting member that takes in from the light inlet the light that passes through or is reflected from the color filters and guides it in a prescribed direction, wherein at least one of the multiple color filters has a central angle extending to the circumference that is different from that for the other filters.
The light illumination device of yet another aspect of the present invention is an illumination device comprising a light source, multiple color filters that have different light permeability or reflectance characteristics and are positioned in a continuous fashion, a drive mechanism constructed such that the multiple color filters that either reflect the light from the light source or allow it to pass through sequentially alternate according to a timing sequence, a lighting member that takes in from the light inlet the light that passes through or is reflected from the color filters and guides it in a prescribed direction, wherein the light inlet has a configuration that extends in the directions parallel to the border between the multiple color filters.
The light illumination device of yet another aspect of the present invention is an illumination device comprising a light source, a rotatable round color wheel that has multiple color filters aligned around the circumference of the color wheel that have different light permeability or reflectance characteristics, wherein when the color wheel rotates, the color filters that either reflect the light from the light source or allow it to pass through sequentially alternate according to a timing sequence, a drive mechanism that drives the color wheel to rotate, and a lighting member that takes in from the light inlet the light that passes through or is reflected from the color filters and guides it in a prescribed direction, wherein the light inlet extends in the directions of the radius of rotation of the color wheel.
In yet another aspect of the present invention, the filter comprises multiple color filters that have different light permeability or reflectance characteristics and are located in a continuous fashion, wherein the area of at least one of the multiple color filters is different from the areas of the other color filters.